1. Field of the Invention
This invention relates to an improved permanent magnet and more particularly to a corrosionproofing treatment for rare earth-iron magnet.
2. Description of the Prior Art
A Sm-Co base magnet has been conventionally used as a high energy product magnet. However, a rare earth-iron base magnet has recently attracted attention because of its low cost, good mechanical processing and higher energy product. Particularly, a magnet having a composition comprising, in terms of atomic %, 8 to 30% R (wherein R is at least one rare earth element including yttrium), 2 to 28% boron with the balance being iron and unavoidable impurities, or 8 to 30% R, 2 to 28% B, more than zero and not exceeding 50% Co, and the balance being Fe with impurities is effective.
About the above rare earth-iron magnets are disclosed in EP Nos. 101 552 A2 and 106 948 A2.
However, the rare earth-iron base magnet is inferior in corrosion and chemical resistance to the one based on Sm-Co, which requires the treatment of the surface thereof. In fact, an attempt has been made to treat the surface with aluminium chromate.